A targeted use of grass-legume mixtures has the potential to increase productivity, resource efficiency and forage quality. The main limitation of using legumes in fertile agricultural systems is insufficient persistence in the sward due to competitive exclusion from the companion grass. In northern areas winter survival becomes an additional obstacle. Monocultures and 11 different mixtures of two grass species and two legume species, differing in temporal development, were established at Korpa Experimental Station (64°N) in spring 2008. The four agronomic species used were timothy (Phleum pratense) (fast establishing), meadow fescue (Festuca pratensis) (persistent), red clover (Trifolium pratense) (fast establishing) and white clover (Trifolium repens) (persistent), and they received three levels of N fertilizer (20, 70 and 220 kg N ha^-1). Mixtures were more productive and showed greater yield stability over time than their individual components in monoculture irrespective of fertiliser treatment. The equal stand mixtures yielded, on average over four years, 69%, 52% and 37% more than could be expected from yield of their components in monoculture at 20, 70 and 220 N, respectively. Mixtures showed significant transgressive over-yielding at 20 and 70 N. A priori selection of plants with different functional traits is a highly promising tool in striving for more sustainable agriculture.